1. Technical Field
The present invention relates to a transmissive diffraction grating and a detection apparatus.
2. Related Art
In the related art, a large number of diffraction gratings which are used in spectroscopic apparatuses such as a Raman spectroscope are of the reflection type. As an example of the reflective diffraction grating, there is the blazed grating of which the cross-section is formed to have a saw-tooth shape (for example, a diagram disclosed in JP-A-2004-354176).
However, the reflective diffraction grating has a problem in that improving the wavelength resolution and widening the wavelength band capable of obtaining high diffraction efficiency are difficult to achieve together. For example, in the blazed diffraction grating, the cross-sectional shape is blazed, and thereby the diffraction efficiency is improved. However, in the blazed diffraction grating, when the grating period is shortened in order to improve the wavelength resolution, a wavelength band capable of obtaining high diffraction efficiency becomes very narrow.
FIG. 12 shows an example of the blazed diffraction grating as a comparative example of the present embodiment. As shown in FIG. 12, it is assumed that a grating period of the blazed diffraction grating is Pa, a wavelength of the incident light is λa, an incidence angle of the incident light is αa, and a diffraction angle of the first-order diffracted light is βa.
First, the wavelength resolution will be described. The wavelength resolution Δβ/Δλ of the diffraction grating is expressed by the following Equation (1). From the following Equation (1), it can be seen that the grating period Pa is made smaller and the diffraction angle βa is made greater in order to increase the wavelength resolution Δβ/Δλ.Δβ/Δλ=1/(Pa·cos βa)  (1)
FIG. 13 shows a characteristic example of the wavelength resolution Δβa/Δλa for the diffraction angle βa when the wavelength λa=633 nm and the grating period Pa=333 nm in Equation (1). In this example, the ratio of the wavelength and the grating period is λa/Pa=1.9. At this time, the diffraction angle βa becomes 72 degrees, and the wavelength resolution Δβa/Δλa is improved to about 0.01.
Next, diffraction efficiency of the first-order diffracted light will be described. In a case of the reflective diffraction grating, the cross-sectional shape thereof is blazed, and thereby it is possible to increase the diffraction efficiency. However, if the grating period Pa becomes smaller in order to increase the wavelength resolution Δβa/Δλa, it is difficult to obtain high diffraction efficiency even if the cross-sectional shape is blazed (The Complete Works of the Latest Optical Diffractive Element Technology, TECHNICAL INFORMATION INSTITUTE CO., LTD, p. 107 to p. 120 (2004)). As such, in the reflective diffraction grating such as the blazed diffraction grating, it is difficult to realize both high wavelength resolution and high diffraction efficiency.
For example, in a spectroscopic apparatus such as a Raman spectroscope, there is a demand for a diffraction grating which achieves both high wavelength resolution and high diffraction efficiency at a wide wavelength band. In Raman spectroscopy, scattering light from a sample is mainly formed by Rayleigh scattering light and Raman scattering light (hereinafter, attention is paid to a Stoke component which has a Raman scattering wavelength λray+Δλ longer than a Rayleigh scattering wavelength λray). In this Raman spectroscopy, there are several problems in terms of practical use. First, the intensity of the Raman scattering light is much weaker than the intensity of the Rayleigh scattering light. Next, in a case of specifying a material by Raman spectroscopy, it is necessary to separate the Raman scattering light scattered from the sample at a wavelength resolution of about 0.5 nm. In addition, there are cases where a wavelength difference between the Rayleigh scattering light and the Raman scattering light is obtained widely up to approximately 100 nm. In consideration of this factor, in the diffraction grating used for Raman spectroscopy, it is necessary to obtain high wavelength resolution of about 0.5 nm from the visible region to the near infrared region (wavelengths 400 nm to 1100 nm). In addition, it is necessary to obtain high diffraction efficiency in a wide wavelength band of approximately 100 nm.
Further, in a case where the optical characteristics of the diffraction grating are considerably dependent on the polarization state of incident light, a detector is only able to incorporate an extremely small portion of scattering light having no deviation in the polarization azimuth and a signal to noise ratio is decreased. Therefore, excessive specifications for the detector are required.